Update ISSUE_TEMPLATE.md

.github/ISSUE_TEMPLATE.md

@@ -1,4 +1,4 @@
-Please search your question on previous issues, stackoverflow or other search engines before you open a new one.
+Please search your question on previous issues, [stackoverflow](https://stackoverflow.com/questions/tagged/lightgbm) or other search engines before you open a new one.
 
 For bugs and unexpected issues, please provide following information, so that we could reproduce on our system.